The use of long-duration current pulses to assess nerve survival

Prediction of Cochlear Implant Fitting by Machine Learning Techniques

OBJECTIVE

This study aimed to evaluate the differences in electrically evoked compound action potential (ECAP) thresholds and postoperative mapping current (T) levels between electrode types after cochlear implantation, the correlation between ECAP thresholds and T levels, and the performance of machine learning techniques in predicting postoperative T levels.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary hospital.

PATIENTS

We reviewed the charts of 124 ears of children with severe-to-profound hearing loss who had undergone cochlear implantation.

INTERVENTIONS

We compared ECAP thresholds and T levels from different electrodes, calculated correlations between ECAP thresholds and T levels, and created five prediction models of T levels at switch-on and 6 months after surgery.

MAIN OUTCOME MEASURES

The accuracy of prediction in postoperative mapping current (T) levels.

RESULTS

The ECAP thresholds of the slim modiolar electrodes were significantly lower than those of the straight electrodes on the apical side. However, there was no significant difference in the neural response telemetry thresholds between the two electrodes on the basal side. Lasso regression achieved the most accurate prediction of T levels at switch-on, and the random forest algorithm achieved the most accurate prediction of T levels 6 months after surgery in this dataset.

CONCLUSION

Machine learning techniques could be useful for accurately predicting postoperative T levels after cochlear implantation in children.

Electrically Evoked Auditory Brainstem Response Using Extracochlear Stimulation at Different Cochlear Sites: A Comparison With Intracochlear Stimulation

OBJECTIVES

The distribution and extent of excitable spiral ganglion neurons (SGNs) have been investigated using the electrically evoked auditory brainstem response (EABR) during preoperative and perioperative periods. In this study, we investigated the EABR with extracochlear stimulation (eEABR) as a preoperative test to estimate these factors.

METHODS

Sixteen male Sprague-Dawley rats were used in this study. Experiments were conducted in nine rats with normal hearing and seven rats that were partially deafened with ouabain treatment. Each experiment involved the following steps: extracochlear stimulating electrode placement at three different sites along the axis of the cochlea and eEABR recordings; cochleostomy and four-channel intracochlear array implantation, followed by EABR recordings with various electrode pair combinations; and after electrophysiological measurements, harvest of the cochleae for histopathological evaluation. The slope characteristics of the amplitude growth function measured from eEABR and EABR, frequency-specific auditory thresholds, and the density of SGNs were compared.

RESULTS

Similar trends were observed in slope changes on different sites of stimulation with both types of stimulation in normal-hearing animals-specifically, a monotonically increasing slope with increasing distance between bipolar pairs. In addition, eEABR slopes showed significant correlations with EABR slopes when the expected cochlear regions of stimulation were similar in normal-hearing animals. In partially deaf animals, the auditory thresholds at several frequencies had a significant correlation with the eEABR slopes of each extracochlear electrode at the apical, middle, and basal cochlear positions. This indicated that increasing the regions of cochlear stimulation had a differential impact on eEABR slopes, depending on the neural conditions.

CONCLUSION

Our results indicated that eEABR slopes showed significant spatial correlations with the functionality of the auditory nerve. Therefore, eEABR tests at various cochlear positions might be used for estimating the extent of excitable SGNs in cochlear implant candidates prior to implantation.

Facilitation properties in electrically evoked compound action potentials depending on spatial location and on threshold

Spiral ganglion neurons (SGNs) facilitation properties can be recorded utilizing electrically evoked compound action potential (ECAP). While intracochlear variation of the ECAP threshold in relation to its electrode channel is reported, no study investigated its impact on facilitation. In this study, we quantified intracochlear variation of the facilitation properties in cochlear implants (CI) using ECAPs. We hypothesized that the facilitation effect is dependent on the electrode channel and its ECAP threshold. Therefore, ECAPs were recorded in 23 CI subjects. For each subject, five default (channel-derived) and up to two additional (threshold-derived) stimulation sites were defined. Facilitation was quantified by the paradigm introduced by (Hey et al., 2017) with optimized parameter settings. For each channel the maximum facilitated amplitude was determined by a series of ECAP measurements. A linear mixed-effects model was used to investigate the impact of the electrode channel and ECAP threshold on the maximum facilitated amplitude. The maximum facilitated amplitude was found to be dependent on the ECAP threshold and independent on the electrode channel. We conclude that the facilitation paradigm is a useful and feasible tool to gain local information on the SGNs temporal processing patterns.

Relationship between electrically evoked compound action potential thresholds and behavioral T-levels in implanted children with cochlear nerve deficiency

It is challenging to program children with cochlear nerve deficiency (CND) due to limited auditory and speech abilities or concurrent neurological deficits. Electrically evoked compound action potential (ECAP) thresholds have been widely used by many audiologists to help cochlear implant programming for children who cannot cooperate with behavioral testing. However, the relationship between ECAP thresholds and behavioral levels of cochlear nerve in children with CND remains unclear. This study aimed to investigate how well ECAP thresholds are related to behavioral thresholds in the MAP for children with CND. This study included 29 children with CND who underwent cochlear implantation. For each participant, ECAP thresholds and behavioral T-levels were measured at three electrode locations across the electrode array post-activation. The relationship between ECAP thresholds and behavioral T-levels was analyzed using Pearson's correlation coefficient. The results showed that ECAP thresholds were significantly correlated with behavioral T-levels at the basal, middle, and apical electrodes. ECAP thresholds were equal to or higher than the behavioral T-levels for all tested electrodes, and fell within MAP's dynamic range for approximately 90% of the tested electrodes. Moreover, the contour of the ECAP thresholds was similar to the contour of T-levels across electrodes for most participants. ECAP thresholds can help audiologists select stimulation levels more efficiently for children with CND who cannot provide sufficient behavioral response.

Characteristics of electrically evoked auditory brainstem response in children with large vestibular aqueduct syndrome after cochlear implantation

BACKGROUND

The vestibular aqueduct (VA) width may affect the auditory conduction pathway of large vestibular aqueduct syndrome (LVAS) children.

OBJECTIVE

Analyzing the electrically evoked auditory brainstem response (EABR) after cochlear implantation (CI) in severe to profound sensorineural hearing loss (SNHL) children with LVAS.

MATERIALS AND METHODS

Fifty-four children with SNHL who received CI were selected, including 21 children with LVAS and 33 children without inner ear malformations (IEMs). The VA width was measured in LVAS children. The post-operative EABRs were recorded in all children.

RESULTS

For the LVAS group, the VA width was positively correlated with wave III (eIII) latency of EABR at the No. 2 electrode (E2), E5, E8, and E11, the VA width was positively correlated with wave V (eV) latency of EABR at E2, E5, E8, and E11. Only the eV latency of E2 showed a significant difference between the two groups.

CONCLUSION AND SIGNIFICANCE

SNHL children with LVAS and no IEMs have similar conduction of the auditory pathway. In the LVAS group, the conduction of the auditory pathway becomes better with the decrease of VA width. Rational use of post-operative EABRs and the measurement of anatomical parameters can effectively assess the development of part auditory pathways in LVAS children, which may be helpful in predicting post-operative speech and hearing recovery.

Effect of Increasing Pulse Phase Duration on Neural Responsiveness of the Electrically Stimulated Cochlear Nerve

OBJECTIVES

The aim of this study is to (1) investigate the effects of increasing the pulse phase duration (PPD) on the neural response of the electrically stimulated cochlear nerve (CN) in children with CN deficiency (CND) and (2) compare the results from the CND population to those measured in children with normal-sized CNs.

DESIGN

Study participants included 30 children with CND and 30 children with normal-sized CNs. All participants used a Cochlear Nucleus device in the test ear. For each subject, electrically evoked compound action potential (eCAP) input/output (I/O) functions evoked by single biphasic pulses with different PPDs were recorded at three electrode locations across the electrode array. PPD durations tested in this study included 50, 62, 75, and 88 μsec/phase. For each electrode tested for each study participant, the amount of electrical charge corresponding to the maximum comfortable level measured for the 88 μsec PPD was used as the upper limit of stimulation. The eCAP amplitude measured at the highest electrical charge level, the eCAP threshold (i.e., the lowest level that evoked an eCAP), and the slope of the eCAP I/O function were measured. Generalized linear mixed effect models with study group, electrode location, and PPD as the fixed effects and subject as the random effect were used to compare these dependent variables measured at different electrode locations and PPDs between children with CND and children with normal-sized CNs.

RESULTS

Children with CND had smaller eCAP amplitudes, higher eCAP thresholds, and smaller slopes of the eCAP I/O function than children with normal-sized CNs. Children with CND who had fewer electrodes with a measurable eCAP showed smaller eCAP amplitudes and flatter eCAP I/O functions than children with CND who had more electrodes with eCAPs. Increasing the PPD did not show a statistically significant effect on any of these three eCAP parameters in the two subject groups tested in this study.

CONCLUSIONS

For the same amount of electrical charge, increasing the PPD from 50 to 88 μsec for a biphasic pulse with a 7 μsec interphase gap did not significantly affect CN responsiveness to electrical stimulation in human cochlear implant users. Further studies with different electrical pulse configurations are warranted to determine whether evaluating the eCAP sensitivity to changes in the PPD can be used as a testing paradigm to estimate neural survival of the CN for individual cochlear implant users.

The Effects of GJB2 or SLC26A4 Gene Mutations on Neural Response of the Electrically Stimulated Auditory Nerve in Children

OBJECTIVES

This study aimed to (1) investigate the effect of GJB2 and SLC26A4 gene mutations on auditory nerve function in pediatric cochlear implant users and (2) compare their results with those measured in implanted children with idiopathic hearing loss.

DESIGN

Participants included 20 children with biallelic GJB2 mutations, 16 children with biallelic SLC26A4 mutations, and 19 children with idiopathic hearing loss. All subjects except for two in the SLC26A4 group had concurrent Mondini malformation and enlarged vestibular aqueduct. All subjects used Cochlear Nucleus devices in their test ears. For each subject, electrophysiological measures of the electrically evoked compound action potential (eCAP) were recorded using both anodic- and cathodic-leading biphasic pulses. Dependent variables (DVs) of interest included slope of eCAP input/output (I/O) function, the eCAP threshold, and eCAP amplitude measured at the maximum comfortable level (C level) of the anodic-leading stimulus (i.e., the anodic C level). Slopes of eCAP I/O functions were estimated using statistical modeling with a linear regression function. These DVs were measured at three electrode locations across the electrode array. Generalized linear mixed effect models were used to evaluate the effects of study group, stimulus polarity, and electrode location on each DV.

RESULTS

Steeper slopes of eCAP I/O function, lower eCAP thresholds, and larger eCAP amplitude at the anodic C level were measured for the anodic-leading stimulus compared with the cathodic-leading stimulus in all subject groups. Children with GJB2 mutations showed steeper slopes of eCAP I/O function and larger eCAP amplitudes at the anodic C level than children with SLC26A4 mutations and children with idiopathic hearing loss for both the anodic- and cathodic-leading stimuli. In addition, children with GJB2 mutations showed a smaller increase in eCAP amplitude when the stimulus changed from the cathodic-leading pulse to the anodic-leading pulse (i.e., smaller polarity effect) than children with idiopathic hearing loss. There was no statistically significant difference in slope of eCAP I/O function, eCAP amplitude at the anodic C level, or the size of polarity effect on all three DVs between children with SLC26A4 mutations and children with idiopathic hearing loss. These results suggested that better auditory nerve function was associated with GJB2 but not with SLC26A4 mutations when compared with idiopathic hearing loss. In addition, significant effects of electrode location were observed for slope of eCAP I/O function and the eCAP threshold.

CONCLUSIONS

GJB2 and SLC26A4 gene mutations did not alter polarity sensitivity of auditory nerve fibers to electrical stimulation. The anodic-leading stimulus was generally more effective in activating auditory nerve fibers than the cathodic-leading stimulus, despite the presence of GJB2 or SLC26A4 mutations. Patients with GJB2 mutations appeared to have better functional status of the auditory nerve than patients with SLC26A4 mutations who had concurrent Mondini malformation and enlarged vestibular aqueduct and patients with idiopathic hearing loss.

The Effect of Pulse Polarity on Neural Response of the Electrically Stimulated Cochlear Nerve in Children With Cochlear Nerve Deficiency and Children With Normal-Sized Cochlear Nerves

OBJECTIVE

This study aimed to (1) investigate the effect of pulse polarity on neural response of the electrically stimulated cochlear nerve in children with cochlear nerve deficiency (CND) and children with normal-sized cochlear nerves and (2) compare the size of the pulse polarity effect between these two subject groups.

DESIGN

The experimental and control group included 31 children with CND and 31 children with normal-sized cochlear nerves, respectively. For each study participant, evoked compound action potential (eCAP) input/output (I/O) functions for anodic-leading and cathodic-leading biphasic stimuli were measured at three electrode locations across the electrode array. The dependent variables of interest included the eCAP amplitude measured at the maximum comfortable level of the anodic stimulus, the lowest level that could evoke an eCAP (i.e., the eCAP threshold), the slope of the eCAP I/O function estimated based on linear regression, the negative-peak (i.e., N1) latency of the eCAP, as well as the size of the pulse polarity effect on these eCAP measurements. Generalized linear mixed effect models were used to compare the eCAP amplitude, the eCAP threshold, the slope of the eCAP I/O function, and the N1 latency evoked by the anodic-leading stimulus with those measured for the cathodic-leading stimulus for children with CND and children with normal-sized cochlear nerves. Generalized linear mixed effect models were also used to compare the size of the pulse polarity effect on the eCAP between these two study groups. The one-tailed Spearman correlation test was used to assess the potential correlation between the pulse phase duration and the difference in N1 latency measured for different pulse polarities.

RESULTS

Compared with children who had normal-sized cochlear nerves, children with CND had reduced eCAP amplitudes, elevated eCAP thresholds, flatter eCAP I/O functions, and prolonged N1 latencies. The anodic-leading stimulus led to higher eCAP amplitudes, lower eCAP thresholds, and shorter N1 latencies than the cathodic-leading stimulus in both study groups. Steeper eCAP I/O functions were recorded for the anodic-leading stimulus than those measured for the cathodic-leading stimulus in children with CND, but not in children with normal-sized cochlear nerves. Group differences in the size of the pulse polarity effect on the eCAP amplitude, the eCAP threshold, or the N1 latency were not statistically significant.

CONCLUSIONS

Similar to the normal-sized cochlear nerve, the hypoplastic cochlear nerve is more sensitive to the anodic-leading than to the cathodic-leading stimulus. Results of this study do not provide sufficient evidence for proving the idea that the pulse polarity effect can provide an indication for local neural health.

Electrophysiological Estimates of the Electrode-Neuron Interface Differ Between Younger and Older Listeners With Cochlear Implants

OBJECTIVES

The primary objective of this study was to quantify differences in evoked potential correlates of spiral ganglion neuron (SGN) density between younger and older individuals with cochlear implants (CIs) using the electrically evoked compound action potential (ECAP). In human temporal bone studies and in animal models, SGN density is the lowest in older subjects and in those who experienced long durations of deafness during life. SGN density also varies as a function of age at implantation and hearing loss etiology. Taken together, it is likely that younger listeners who were deafened and implanted during childhood have denser populations of SGNs than older individuals who were deafened and implanted later in life. In animals, ECAP amplitudes, amplitude growth function (AGF) slopes, and their sensitivity to stimulus interphase gap (IPG) are predictive of SGN density. The authors hypothesized that younger listeners who were deafened and implanted as children would demonstrate larger ECAP amplitudes, steeper AGF slopes, and greater IPG sensitivity than older, adult-deafened and implanted listeners.

DESIGN

Data were obtained from 22 implanted ears (18 individuals). Thirteen ears (9 individuals) were deafened and implanted as children (child-implanted group), and nine ears (9 individuals) were deafened and implanted as adults (adult-implanted group). The groups differed significantly on a number of demographic variables that are implicitly related to SGN density: (1) chronological age; (2) age at implantation; and (3) duration of preimplantation hearing loss. ECAP amplitudes, AGF linear slopes, and thresholds were assessed on a subset of electrodes in each ear in response to two IPGs (7 and 30 µsec). Speech recognition was assessed using a medial vowel identification task.

RESULTS

Compared with the adult-implanted listeners, individuals in the child-implanted group demonstrated larger changes in ECAP amplitude when the IPG of the stimulus was increased from 7 to 30 µsec (i.e., greater IPG sensitivity). On average, child-implanted participants also had larger ECAP amplitudes and steeper AGF linear slopes than the adult-implanted participants, irrespective of IPG. IPG sensitivity for AGF linear slope and ECAP threshold did not differ between age groups. Vowel recognition performance was not correlated with any of the ECAP measures assessed in this study.

CONCLUSIONS

The results of this study support the theory that young CI listeners who were deafened and implanted during childhood may have denser neural populations than older listeners who were deafened and implanted as adults. Potential between-group differences in SGN integrity emphasize a need to investigate optimized CI programming parameters for younger and older listeners.

Identifying Cochlear Implant Channels With Relatively Poor Electrode-Neuron Interfaces Using the Electrically Evoked Compound Action Potential

OBJECTIVES

The primary objective of this study was to quantify local (within ear) and global (between ear) variation in the cochlear implant (CI) electrode-neuron interface (ENI) using the electrically evoked compound action potential (ECAP). We tested the hypothesis that, within an ear, ECAP measures can be used to identify channels with presumed good and poor ENIs, which may be influenced by a combination of spiral ganglion neuron (SGN) density, electrode position, and cochlear resistivity. We also hypothesized that ECAP responses would reflect age-related differences in the global quality of the ENI between younger and older listeners who theoretically differ in SGN density.

DESIGN

Data were obtained from 18 implanted ears (13 individuals) with Advanced Bionics HiRes 90K devices. Six participants (8 ears) were adolescents or young adults (age range: 14-32 years), and 7 participants (10 ears) were older adults (age range: 54-88 years). In each ear, single-channel auditory detection thresholds were measured on channels 2 through 15 in response to a spatially focused electrode configuration (steered quadrupolar; focusing coefficient = 0.9). ECAP amplitudes, amplitude growth function (AGF) slopes, and thresholds were assessed on a subset of channels in each ear in response to three interphase gaps (0, 7, and 30 µs). ECAP peak amplitudes were assessed on all channels between 2 and 15. AGFs and ECAP thresholds were measured on the two nonadjacent channels with the lowest and highest focused behavioral thresholds in each ear. ECAP responses were compared across low- and high-threshold channels and between younger and older CI listeners.

RESULTS

Channels that were estimated to interface poorly with the auditory nerve (i.e., high-focused-threshold channels) had steeper ECAP AGF slopes, smaller dynamic ranges, and higher ECAP thresholds than channels with low focused thresholds. Younger listeners had steeper ECAP AGF slopes and larger ECAP peak amplitudes than older listeners. Moreover, younger listeners showed greater interphase gap sensitivity for ECAP amplitude than older listeners.

CONCLUSIONS

ECAP responses may be used to quantify both local (within ear) and global (between ear) variation in the quality of the ENI. Results of this study support future investigation into the use of ECAP responses in site-selection CI programming strategies. The present results also support a growing body of evidence suggesting that adolescents and young adults with CIs may have denser populations of functional SGNs relative to older adults. Potential differences in global SGN integrity between younger and older listeners warrant investigation of optimal CI programming interventions based on their divergent hearing histories.

Electrophysiology and genetic testing in the precision medicine of congenital deafness: A review

BACKGROUND

Congenital hearing loss is remarkably heterogeneous, with over 130 deafness genes and thousands of variants, making for innumerable genotype/phenotype combinations. Understanding both the pathophysiology of hearing loss and molecular site of lesion along the auditory pathway permits for significantly individualized counseling. Electrophysiologic techniques such as electrocochleography (ECochG) and electrically-evoked compound action potentials (eCAP) are being studied to localize pathology and estimate residual cochlear vs. neural health. This review describes the expanding roles of genetic and electrophysiologic evaluation in the precision medicine of congenital hearing loss.The basics of genetic mutations in hearing loss and electrophysiologic testing (ECochG and eCAP) are reviewed, and how they complement each other in the diagnostics and prognostication of hearing outcomes. Used together, these measures improve the understanding of insults to the auditory system, allowing for individualized counseling for CI candidacy/outcomes or other habilitation strategies.

CONCLUSION

Despite tremendous discovery in deafness genes, the effects of individual genes on neural function remain poorly understood. Bridging the understanding between molecular genotype and neural and functional phenotype is paramount to interpreting genetic results in clinical practice. The future hearing healthcare provider must consolidate an ever-increasing amount of genetic and phenotypic information in the precision medicine of hearing loss.

Responsiveness of the Electrically Stimulated Cochlear Nerve in Children With Cochlear Nerve Deficiency

OBJECTIVES

This study aimed to (1) investigate the responsiveness of the cochlear nerve (CN) to a single biphasic-electrical pulse in implanted children with cochlear nerve deficiency (CND) and (2) compare their results with those measured in implanted children with normal-size CNs.

DESIGN

Participants included 23 children with CND (CND1 to CND23) and 18 children with normal-size CNs (S1 to S18). All subjects except for CND1 used Cochlear Nucleus cochlear implants with contour electrode arrays in their test ears. CND1 was implanted with a Cochlear Nucleus Freedom cochlear implant with a straight electrode array in the test ear. For each subject, the CN input/output (I/O) function and the refractory recovery function were measured using electrophysiological measures of the electrically evoked compound action potential (eCAP) at multiple electrode sites across the electrode array. Dependent variables included eCAP threshold, the maximum eCAP amplitude, slope of the I/O function, and time-constants of the refractory recovery function. Slopes of I/O functions were estimated using statistical modeling with a sigmoidal function. Recovery time-constants, including measures of the absolute refractory period and the relative refractory period, were estimated using statistical modeling with an exponential decay function. Generalized linear mixed-effect models were used to evaluate the effects of electrode site on the dependent variables measured in children with CND and to compare results of these dependent variables between subject groups.

RESULTS

The eCAP was recorded at all test electrodes in children with normal-size CNs. In contrast, the eCAP could not be recorded at any electrode site in 4 children with CND. For all other children with CND, the percentage of electrodes with measurable eCAPs decreased as the stimulating site moved in a basal-to-apical direction. For children with CND, the stimulating site had a significant effect on the slope of the I/O functions and the relative refractory period but showed no significant effect on eCAP threshold and the maximum eCAP amplitude. Children with CND had significantly higher eCAP thresholds, smaller maximum eCAP amplitudes, flatter slopes of I/O functions, and longer absolute refractory periods than children with normal-size CNs. There was no significant difference in the relative refractory period measured in these two subject groups.

CONCLUSIONS

In children with CND, the functional status of the CN varied along the length of the cochlea. Compared with children with normal-size CNs, children with CND showed reduced CN responsiveness to electrical stimuli. The prolonged CN absolute refractory period in children with CND might account for, at least partially, the observed benefit of using relatively slow pulse rate in these patients.

Relationship Between Peripheral and Psychophysical Measures of Amplitude Modulation Detection in Cochlear Implant Users

OBJECTIVE

This study investigates the relationship between electrophysiological and psychophysical measures of amplitude modulation (AM) detection. Prior studies have reported both measures of AM detection recorded separately from cochlear implant (CI) users and acutely deafened animals, but no study has made both measures in the same CI users. Animal studies suggest a progressive loss of high-frequency encoding as one ascends the auditory pathway from the auditory nerve to the cortex. Because the CI speech processor uses the envelope of an ongoing acoustic signal to modulate pulse trains that are subsequently delivered to the intracochlear electrodes, it is of interest to explore auditory nerve responses to modulated stimuli. In addition, psychophysical AM detection abilities have been correlated with speech perception outcomes. Thus, the goal was to explore how the auditory nerve responds to AM stimuli and to relate those physiologic measures to perception.

DESIGN

Eight patients using Cochlear Ltd. Implants participated in this study. Electrically evoked compound action potentials (ECAPs) were recorded using a 4000 pps pulse train that was sinusoidally amplitude modulated at 125, 250, 500, and 1000 Hz rates. Responses were measured for each pulse over at least one modulation cycle for an apical, medial, and basal electrode. Psychophysical modulation detection thresholds (MDTs) were also measured via a three-alternative forced choice, two-down, one-up adaptive procedure using the same modulation frequencies and electrodes.

RESULTS

ECAPs were recorded from individual pulses in the AM pulse train. ECAP amplitudes varied sinusoidally, reflecting the sinusoidal variation in the stimulus. A modulated response amplitude (MRA) metric was calculated as the difference in the maximal and minimum ECAP amplitudes over the modulation cycles. MRA increased as modulation frequency increased, with no apparent cutoff (up to 1000 Hz). In contrast, MDTs increased as the modulation frequency increased. This trend is inconsistent with the physiologic measures. For a fixed modulation frequency, correlations were observed between MDTs and MRAs; this trend was evident at all frequencies except 1000 Hz (although only statistically significant for 250 and 500 Hz AM rates), possibly an indication of central limitations in processing of high modulation frequencies. Finally, peripheral responses were larger and psychophysical thresholds were lower in the apical electrodes relative to basal and medial electrodes, which may reflect better cochlear health and neural survival evidenced by lower preoperative low-frequency audiometric thresholds and steeper growth of neural responses in ECAP amplitude growth functions for apical electrodes.

CONCLUSIONS

Robust ECAPs were recorded for all modulation frequencies tested. ECAP amplitudes varied sinusoidally, reflecting the periodicity of the modulated stimuli. MRAs increased as the modulation frequency increased, a trend we attribute to neural adaptation. For low modulation frequencies, there are multiple current steps between the peak and valley of the modulation cycle, which means successive stimuli are more similar to one another and neural responses are more likely to adapt. Higher MRAs were correlated with lower psychophysical thresholds at low modulation frequencies but not at 1000 Hz, implying a central limitation to processing of modulated stimuli.

Effects of Stimulus Polarity and Artifact Reduction Method on the Electrically Evoked Compound Action Potential

OBJECTIVE

Previous research from our laboratory comparing electrically evoked compound action potential (ECAP) artifact reduction methods has shown larger amplitudes and lower thresholds with cathodic-leading forward masking (CathFM) than with alternating polarity (AltPol). One interpretation of this result is that the anodic-leading phase used with AltPol elicits a less excitatory response (in contrast to results from recent studies with humans), which when averaged with responses to cathodic-leading stimuli, results in smaller amplitudes. Another interpretation is that the latencies of the responses to anodic- and cathodic-leading pulses differ, which when averaged together, result in smaller amplitudes than for either polarity alone due to temporal smearing. The purpose of this study was to separate the effects of stimulus polarity and artifact reduction method to determine the relative effects of each.

DESIGN

This study used a within-subjects design. ECAP growth functions were obtained using CathFM, anodic-leading forward masking (AnodFM), and AltPol for 23 CI recipients (N = 13 Cochlear and N = 10 Advanced Bionics). N1 latency, amplitude, slope of the amplitude growth function, and threshold were compared across methods. Data were analyzed separately for each manufacturer due to inherent differences between devices.

RESULTS

N1 latencies were significantly shorter for AnodFM than for CathFM and AltPol for both Cochlear and Advanced Bionics participants. Amplitudes were larger for AnodFM than for either CathFM or AltPol for Cochlear recipients; amplitude was not significantly different across methods for Advanced Bionics recipients. Slopes were shallowest for CathFM for Cochlear subjects, but were not significantly different among methods for Advanced Bionics subjects. Thresholds with AltPol were significantly higher than both FM methods for Cochlear recipients; there was no difference in threshold across methods for the Advanced Bionics recipients.

CONCLUSIONS

For Cochlear devices, the smaller amplitudes and higher thresholds observed for AltPol seem to be the result of latency differences between polarities. These results suggest that AltPol is not ideal for managing stimulus artifact for ECAP recordings. For the Advanced Bionics group, there were no significant differences among methods for amplitude, slope, or threshold, which suggests that polarity and artifact reduction method have little influence in these devices. We postulate that polarity effects are minimized for symmetrical biphasic pulses that lack an interphase gap, such as those used with Advanced Bionics devices; however, this requires further investigation.

A new approach for the determination of ECAP thresholds

BACKGROUND

Electrically evoked compound action potentials (ECAPs) of the auditory nerve are routinely recorded for testing the cochlear implant integrity and its functional connection to the auditory system. The response thresholds derived from ECAP recordings are widely used as a helpful guide in the fitting of the dynamic range of electric stimulation, although they may not always predict the behavioral thresholds of individuals well. Conventionally, this threshold is based on the identification of a minimum N peak and maximum P peak and linear extrapolation of the resulting amplitude growth function (AGF). As an alternative, a new procedure involving numeric signal processing and requiring less user intervention is presented here. Data acquisition: In 12 adults implanted with MED-EL FLEX28 electrodes, two series of ECAPs were recorded immediately after implantation: (i) a full profile involving all 12 channels across the whole stimulus range in steps of 200 current units and (ii) a high resolution section (20 records in the immediate neighborhood of the threshold) of the AGF in one selected channel. Data treatment: It was observed that N and P wave latencies do not depend on stimulus intensity. Fixed time windows were hence defined for stimulus plus noise and noise alone regions. In these windows, the variance of the compound signal representing response and noise is extracted, whereas the noise variance is extracted from the tail of the curve following this time window. The base line is corrected by fitting an exponential function to reduce stimulus or amplifier artifacts. The response threshold is then derived from the response to noise ratio which should exceed the limit of 6 dB.

RESULTS

The ECAP thresholds obtained from the new procedure coincide well with those determined by the conventional linear extrapolation of the AGF and they correlate to a greater degree with psychometric thresholds than the existing approach.

CONCLUSIONS

The new ECAP algorithm looks promising and may reduce the need for user intervention in determining thresholds.

The Electrically Evoked Compound Action Potential: From Laboratory to Clinic

The electrically evoked compound action potential (eCAP) represents the synchronous firing of a population of electrically stimulated auditory nerve fibers. It can be directly recorded on a surgically exposed nerve trunk in animals or from an intra-cochlear electrode of a cochlear implant. In the past two decades, the eCAP has been widely recorded in both animals and clinical patient populations using different testing paradigms. This paper provides an overview of recording methodologies and response characteristics of the eCAP, as well as its potential applications in research and clinical situations. Relevant studies are reviewed and implications for clinicians are discussed.

Intracochlear Recordings of Acoustically and Electrically Evoked Potentials in Nucleus Hybrid L24 Cochlear Implant Users and Their Relationship to Speech Perception

The Hybrid cochlear implant (CI) has been developed for individuals with high frequency hearing loss who retain good low frequency hearing. Outcomes have been encouraging but individual variability is high; the health of the cochlea and the auditory nerve may be important factors driving outcomes. Electrically evoked compound action potentials (ECAPs) reflect the response of the auditory nerve to electrical stimulation while electrocochleography (ECochG) reflects the response of the cochlear hair cells and auditory nerve to acoustic stimulation. In this study both ECAPs and ECochG responses were recorded from Nucleus Hybrid L24 CI users. Correlations between these two measures of peripheral auditory function and speech perception are reported. This retrospective study includes data from 25 L24 CI users. ECAPs and ECochG responses were recorded from an intracochlear electrode using stimuli presented at or near maximum acceptable loudness levels. Speech perception was assessed using Consonant-Nucleus-Consonant (CNC) word lists presented in quiet and AzBio sentences presented at a +5 dB signal-to-noise ratio in both the combined acoustic and electric (A+E) and electric (E) alone listening modes. Acoustic gain was calculated by subtracting these two scores. Correlations between these physiologic and speech perception measures were then computed. ECAP amplitudes recorded from the most apical electrode were significantly correlated with CNC scores measured in the E alone (r = 0.56) and A+E conditions (r = 0.64), but not with performance on the AzBio test. ECochG responses recorded using the most apical electrode in the intracochlear array but evoked using a 500 Hz tone burst were not correlated with either the scores on the CNC or AzBio tests. However, ECochG amplitude was correlated with a composite metric relating the additional benefit of acoustic gain in noise relative to quiet conditions (r = 0.67). Both measures can be recorded from Hybrid L24 CI users and both ECAP and ECochG measures may result in more complete characterization of speech perception outcomes than either measure alone.

Suprathreshold compound action potential amplitude as a measure of auditory function in cochlear implant users

Electrically evoked compound action potential (eCAP) amplitudes elicited at suprathreshold levels were assessed as a measure of the effectiveness of cochlear implant (CI) stimulation. Twenty-one individuals participated; one was excluded due to facial stimulation during eCAP testing. For each participant, eCAPs were elicited with stimulation from seven electrodes near the upper limit of the individual's electrical dynamic range. A reduced-channel CI program was created using those same seven electrodes, and participants performed a vowel discrimination task. Consistent with previous reports, eCAP amplitudes varied across tested electrodes; the profiles were unique to each individual. In 6 subjects (30%), eCAP amplitude variability was partially explained by the impedance of the recording electrode. The remaining amplitude variability within subjects, and the variability observed across subjects could not be explained by recording electrode impedance. This implies that other underlying factors, such as variations in neural status across the array, are responsible. Across-site mean eCAP amplitude was significantly correlated with vowel discrimination scores (r2 = 0.56). A single eCAP amplitude measured from the middle of the array was also significantly correlated with vowel discrimination, but the correlation was weaker (r2 = 0.37), though not statistically different from the across-site mean. Normalizing each eCAP amplitude by its associated recording electrode impedance did not improve the correlation with vowel discrimination (r2 = 0.52). Further work is needed to assess whether combining eCAP amplitude with other measures of the electrode-neural interface and/or with more central measures of auditory function provides a more complete picture of auditory function in CI recipients.

Assessing the Electrode-Neuron Interface with the Electrically Evoked Compound Action Potential, Electrode Position, and Behavioral Thresholds

Variability in speech perception scores among cochlear implant listeners may largely reflect the variable efficacy of implant electrodes to convey stimulus information to the auditory nerve. In the present study, three metrics were applied to assess the quality of the electrode-neuron interface of individual cochlear implant channels: the electrically evoked compound action potential (ECAP), the estimation of electrode position using computerized tomography (CT), and behavioral thresholds using focused stimulation. The primary motivation of this approach is to evaluate the ECAP as a site-specific measure of the electrode-neuron interface in the context of two peripheral factors that likely contribute to degraded perception: large electrode-to-modiolus distance and reduced neural density. Ten unilaterally implanted adults with Advanced Bionics HiRes90k devices participated. ECAPs were elicited with monopolar stimulation within a forward-masking paradigm to construct channel interaction functions (CIF), behavioral thresholds were obtained with quadrupolar (sQP) stimulation, and data from imaging provided estimates of electrode-to-modiolus distance and scalar location (scala tympani (ST), intermediate, or scala vestibuli (SV)) for each electrode. The width of the ECAP CIF was positively correlated with electrode-to-modiolus distance; both of these measures were also influenced by scalar position. The ECAP peak amplitude was negatively correlated with behavioral thresholds. Moreover, subjects with low behavioral thresholds and large ECAP amplitudes, averaged across electrodes, tended to have higher speech perception scores. These results suggest a potential clinical role for the ECAP in the objective assessment of individual cochlear implant channels, with the potential to improve speech perception outcomes.

Development of brainstem-evoked responses in congenital auditory deprivation

To compare the development of the auditory system in hearing and completely acoustically deprived animals, naive congenitally deaf white cats (CDCs) and hearing controls (HCs) were investigated at different developmental stages from birth till adulthood. The CDCs had no hearing experience before the acute experiment. In both groups of animals, responses to cochlear implant stimulation were acutely assessed. Electrically evoked auditory brainstem responses (E-ABRs) were recorded with monopolar stimulation at different current levels. CDCs demonstrated extensive development of E-ABRs, from first signs of responses at postnatal (p.n.) day 3 through appearance of all waves of brainstem response at day 8 p.n. to mature responses around day 90 p.n.. Wave I of E-ABRs could not be distinguished from the artifact in majority of CDCs, whereas in HCs, it was clearly separated from the stimulus artifact. Waves II, III, and IV demonstrated higher thresholds in CDCs, whereas this difference was not found for wave V. Amplitudes of wave III were significantly higher in HCs, whereas wave V amplitudes were significantly higher in CDCs. No differences in latencies were observed between the animal groups. These data demonstrate significant postnatal subcortical development in absence of hearing, and also divergent effects of deafness on early waves II–IV and wave V of the E-ABR.

Identifying cochlear implant channels with poor electrode-neuron interfaces: electrically evoked auditory brain stem responses measured with the partial tripolar configuration

OBJECTIVES

The goal of this study was to compare cochlear implant behavioral measures and electrically evoked auditory brain stem responses (EABRs) obtained with a spatially focused electrode configuration. It has been shown previously that channels with high thresholds, when measured with the tripolar configuration, exhibit relatively broad psychophysical tuning curves. The elevated threshold and degraded spatial/spectral selectivity of such channels are consistent with a poor electrode-neuron interface, defined as suboptimal electrode placement or reduced nerve survival. However, the psychophysical methods required to obtain these data are time intensive and may not be practical during a clinical mapping session, especially for young children. Here, we have extended the previous investigation to determine whether a physiological approach could provide a similar assessment of channel functionality. We hypothesized that, in accordance with the perceptual measures, higher EABR thresholds would correlate with steeper EABR amplitude growth functions, reflecting a degraded electrode-neuron interface.

DESIGN

Data were collected from six cochlear implant listeners implanted with the HiRes 90k cochlear implant (Advanced Bionics). Single-channel thresholds and most comfortable listening levels were obtained for stimuli that varied in presumed electrical field size by using the partial tripolar configuration, for which a fraction of current (σ) from a center active electrode returns through two neighboring electrodes and the remainder through a distant indifferent electrode. EABRs were obtained in each subject for the two channels having the highest and lowest tripolar (σ = 1 or 0.9) behavioral threshold. Evoked potentials were measured with both the monopolar (σ = 0) and a more focused partial tripolar (σ ≥ 0.50) configuration.

RESULTS

Consistent with previous studies, EABR thresholds were highly and positively correlated with behavioral thresholds obtained with both the monopolar and partial tripolar configurations. The Wave V amplitude growth functions with increasing stimulus level showed the predicted effect of shallower growth for the partial tripolar than for the monopolar configuration, but this was observed only for the low-threshold channels. In contrast, high-threshold channels showed the opposite effect; steeper growth functions were seen for the partial tripolar configuration.

CONCLUSIONS

These results suggest that behavioral thresholds or EABRs measured with a restricted stimulus can be used to identify potentially impaired cochlear implant channels. Channels having high thresholds and steep growth functions would likely not activate the appropriate spatially restricted region of the cochlea, leading to suboptimal perception. As a clinical tool, quick identification of impaired channels could lead to patient-specific mapping strategies and result in improved speech and music perception.

The relationship between electrically evoked compound action potential and speech perception: a study in cochlear implant users with short electrode array

OBJECTIVES

To determine the extent to which electrically evoked compound action potential (ECAP) measurements were related with speech perception performance in implant users with a short electrode array and to investigate the relationship between ECAP measures and performance according to specific devices.

DESIGN

Prospective study.

SETTING

Tertiary referral center.

PATIENTS

Seventeen Hybrid cochlear implant users were tested in this study. Subjects were divided into 2 groups: 8 using the Nucleus Hybrid M and 9 using the Nucleus Hybrid RE. In addition, 21 Nucleus Freedom long electrode implant (CI24RE) users also were tested to compare with the results of the old device (CI24M).

MAIN OUTCOME MEASURES

ECAP growth functions were recorded using either an interphase gap (IPG) of 8 or 45 mus. We then calculated the slope of the growth function and changes in sensitivity with IPG. For each subject, these measures were compared with performance on tests of word recognition.

RESULTS

The changes in sensitivity using 2 IPGs showed no correlation with the results of word recognition test in Hybrid cochlear implant users. In contrast, relatively strong correlations have been found between the slope of ECAP growth functions and performance on word recognition test. Additionally, when we separate the results of Hybrid M and RE, the slopes of ECAP growth functions from only Hybrid RE CI recipients were significantly correlated with speech performance. The slopes of ECAP growth function in CI24RE users with long electrode also were significantly correlated with performance. However, comparing between 2 independent correlations in RE devices, correlation was higher in Hybrid RE group.

CONCLUSION

The results presented in this article support the view that slope of the ECAP growth can show significant correlation to performance with a cochlear implant. Furthermore, these results suggest that the strength of the correlation may be related to the specific device. These results suggest that ECAP measures may be useful in developing a test to predict outcomes with the implant.

Reducing blanking artifact in electrically evoked compound action potentials

The main source of distortion in the recording of the electrically evoked compound action potentials is the stimulus artifact. The popular hardware blanking technique tends to reduce this artifact, but generates a blanking artifact as a consequence of the transient state in the amplifier. In this paper we propose two techniques to deal with the blanking artifact. The proposed techniques are combined with conventional and generalized alternating stimulation in order to reduce both stimulus and blanking artifacts in the recording of the evoked potentials. A comparison over 126 evoked potential recordings reveals that the proposed blanking artifact reduction methods improve the quality of electrically evoked compound action potential recordings.

Comparison of electrically evoked compound action potential thresholds and loudness estimates for the stimuli used to program the Advanced Bionics cochlear implant

BACKGROUND

In the mid-1990s, Cochlear Corporation introduced a cochlear implant (CI) to the market that was equipped with hardware that made it possible to record electrically evoked compound action potentials (ECAPs) from CI users of all ages. Over the course of the next decade, many studies were published that compared ECAP thresholds with levels used to program the speech processor of the Nucleus CI. In 2001 Advanced Bionics Corporation introduced the Clarion CII cochlear implant (the Clarion CII internal device is also known as the CII Bionic Ear). This cochlear implant was also equipped with a system that allowed measurement of the ECAP. While a great deal is known about how ECAP thresholds compare with the levels used to program the speech processor of the Nucleus CI, relatively few studies have reported comparisons between ECAP thresholds and the levels used to program the speech processor of the Advanced Bionics CI.

PURPOSE

To explore the relationship between ECAP thresholds and behavioral measures of perceptual dynamic range for the range of stimuli commonly used to program the speech processor of the Advanced Bionics CI.

RESEARCH DESIGN

This prospective and experimental study uses correlational and descriptive statistics to define the relationship between ECAP thresholds and perceptual dynamic range measures.

STUDY SAMPLE

Twelve postlingually deafened adults participated in this study. All were experienced users of the Advanced Bionics CI system.

DATA COLLECTION AND ANALYSIS

ECAP thresholds were recorded using the commercially available SoundWave software. Perceptual measures of threshold (T-level), most comfortable level (M-level), and maximum comfortable level (C-level) were obtained using both "tone bursts" and "speech bursts." The relationship between these perceptual and electrophysiological variables was defined using paired t-tests as well as correlation and linear regression.

RESULTS

ECAP thresholds were significantly correlated with the perceptual dynamic range measures studied; however, correlations were not strong. Analysis of the individual data revealed considerable discrepancy between the contour of ECAP threshold versus electrode function and the behavioral loudness estimates used for programming.

CONCLUSION

ECAP thresholds recorded from Advanced Bionics cochlear implant users always indicated levels where the programming stimulus was audible for the listener. However, the correlation between ECAP thresholds and M-levels (the primary metric used to program the speech processor of the Advanced Bionics CI), while statistically significant, was quite modest. If programming levels are to be determined on the basis of ECAP thresholds, care should be taken to ensure that stimulation is not uncomfortably loud, particularly on the basal electrodes in the array.

Enhanced survival of spiral ganglion cells after cessation of treatment with brain-derived neurotrophic factor in deafened guinea pigs

Exogenous delivery of neurotrophic factors into the cochlea of deafened animals rescues spiral ganglion cells (SGCs) from degeneration. To be clinically relevant for human cochlear implant candidates, the protective effect of neurotrophins should persist after cessation of treatment and the treated SGCs should remain functional. In this study, the survival and functionality of SGCs were investigated after temporary treatment with brain-derived neurotrophic factor (BDNF). Guinea pigs in the experimental group were deafened, and 2 weeks later, the right cochleae were implanted with an electrode array and drug delivery cannula. BDNF was administered to the implanted cochleae during a 4-week period via a mini-osmotic pump. After completion of the treatment, the osmotic pumps were removed. Two weeks later, the animals were killed and the survival of SGCs was analyzed. To monitor the functionality of the auditory nerve, electrically evoked auditory brainstem responses (eABRs) were recorded in awake animals throughout the experiment. BDNF treatment resulted in enhanced survival of SGCs 2 weeks after cessation of the treatment and prevented the decreases in size and circularity that are seen in the untreated contralateral cochleae. The amplitude of the suprathreshold eABR response in BDNF-treated animals was significantly larger than in deafened control animals and comparable to that in normal-hearing control animals. The amplitude in the BDNF-treated group did not decrease significantly after cessation of treatment. The eABR latency in BDNF-treated animals was longer than normal and comparable to that in deafened control animals. These morphological and functional findings demonstrate that neurotrophic intervention had a lasting effect, which is promising for future clinical application of neurotrophic factors in implanted human cochleae.

Binaural interactions of electrically and acoustically evoked responses recorded from the inferior colliculus of guinea pigs

Binaural interactions within the inferior colliculus (IC) elicited by electric and acoustic stimuli were investigated in this study. Using a guinea pig model, binaural acoustic stimuli were presented with different time delays, as were combinations of binaural electric and acoustic stimuli. Averaged evoked potentials were measured using electrodes inserted into the central nucleus of the IC to obtain the binaural interaction component (BIC), computed by subtracting the sum of the two monaural responses from the binaural response. The BICs to acoustic-acoustic stimulation and electric-acoustic stimulation were found to be similar. The BIC amplitude increased with stimulus intensity, but the shapes of the delay functions were similar across the levels tested. The gross-potential data are thus consistent with the thesis that the central auditory system processes binaural electric and acoustic stimuli in a similar manner. These results suggest that the binaural auditory system can process combinations of electric and acoustic stimulation presented across ears and that evoked gross potentials may be used to measure such interaction.

Neurotrophic factors and neural prostheses: potential clinical applications based upon findings in the auditory system

Spiral ganglion neurons (SGNs) are the target cells of the cochlear implant, a neural prosthesis designed to provide important auditory cues to severely or profoundly deaf patients. The ongoing degeneration of SGNs that occurs following a sensorineural hearing loss is, therefore, considered a limiting factor in cochlear implant efficacy. We review neurobiological techniques aimed at preventing SGN degeneration using exogenous delivery of neurotrophic factors. Application of these proteins prevents SGN degeneration and can enhance neurite outgrowth. Furthermore, chronic electrical stimulation of SGNs increases neurotrophic factor-induced survival and is correlated with functional benefits. The application of neurotrophic factors has the potential to enhance the benefits that patients can derive from cochlear implants; moreover, these techniques may be relevant for use with neural prostheses in other neurological conditions.

Cochlear expression of a dominant-negative GJB2R75W construct delivered through the round window membrane in mice

Development of a gene-delivery method to the inner ear is an essential step for studies of hearing function and gene therapy. Application of liposomes or adenoviral vectors onto the intact round window membrane (RWM) offers the possibility of atraumatic exogenous gene transfer. GJB2 encodes the gap junction protein Connexin26, which plays a crucial role in potassium recycling in the inner ear. The R75W allele of GJB is a well-characterized mutation that causes deafness at the DFNA3 through a dominant-negative mechanism of action. In this study, a plasmid vector, pGJB2(R75W)-eGFP, was lipocomplexed with N-[1-(2,3-Dioleoloxy)propyl]N,N,N-trimethylammonium methylsulfate: cholesterol and applied onto mouse RWM. At 3 days (3d) post-treatment, immunohistochemistry demonstrated GJB2(R75W)-eGFP transgene expression in the cochlea in: inner and outer pillar cells, outer hair cells, Claudius cells and, in the spiral limbus and ligament. Significant hearing loss was detected by auditory brainstem response testing after 1, 2 and 3d post-treatment; hearing levels returned to control levels at 5d post-treatment. These data confirm that GJB2(R75W) induces functional impairment in the mature cochlea through a dominant negative effect, and importantly, that RWM application of exogenous genes is a feasible method to test their impact on hearing.

Protection and regrowth of the auditory nerve after deafness: neurotrophins, antioxidants and depolarization are effective in vivo

Deafness, with loss of sensory (hair) cells, results in progressive pathophysiological changes ending in the degeneration of most auditory nerve neurons. It is now possible to consider these events in the broader context of anti-apoptotic survival factors in the peripheral and central nervous system. One consequence of deafferentation of a neuron is the loss of neurotrophins that can lead to a change in oxidative state (formation of free radicals), changes in intracellular Ca(2+), and an up-regulation of apoptotic genes. Interventions that can modify availability of neurotrophins, [Ca(2+)](I), and/or free radical formation or their destructive effects, may preserve the auditory nerve. Some interventions (neurotrophins) may also lead to a regrowth of neurites. Studies in this area are of basic value and also of immediate clinical interest for the application of the cochlear prosthesis to the severe and profoundly deaf, since the benefits of this prosthesis are directly dependent on auditory nerve survival and the proximity of stimulating electrode to neuron. We, and others, have found that auditory nerve degeneration can be prevented by chronic electrical stimulation. We have demonstrated in vivo that this effect can be blocked by tetrodotoxin, thus indicating that propagated action potentials are a necessary condition, and by verapamil (Ca(2+) channel blocker), supporting in vitro studies by others, indicating that L-type Ca(2+) channels are necessary for stimulation-induced rescue of the deafferented auditory nerve. The intensities of electrical stimulation required for rescue are at levels sufficient to express the intermediate-early gene c-fos which can initiate transcription of anti-apoptotic genes and pathways, and up-regulate expression of neurotrophins that may act in an autocrine manner to protect the nerve from death. We, and others, have found that chronic local delivery (osmotic pump and microcannulation of the inner ear fluid spaces) of individual neurotrophins and cocktails of factors can also enhance survival of the deafferented nerve, and some can also initiate a regrowth of degenerated peripheral processes of the nerve into the region of the destroyed sensory epitheliae. Recently, we have shown that this rescue can occur with delayed intervention, after degeneration of some neurons has begun, more closely mimicking the human clinical situation. Finally, we have shown that interventions with antioxidants may also be effective in preventing pathophysiological changes of the auditory nerve following deafness. These studies in the auditory periphery support the 'neurotrophic factor hypothesis' as proposed as a general mechanism underlying neurodegenerative and age-related pathology of the central nervous system. Additional animal studies can yield a rational scientific basis to justify human trials, with a goal to maintain auditory cell survival and initiate and direct fiber growth to the next generation of prosthesis. Intimate contact between electrode and a dense population of auditory neurons should greatly enhance the benefits of these devices for the profoundly deaf.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

The neuronal response to electrical constant-amplitude pulse train stimulation: additive Gaussian noise

Experimental results from humans and animals show that electrically evoked compound action potential (EAP) responses to constant-amplitude pulse train stimulation can demonstrate an alternating pattern, due to the combined effects of highly synchronized responses to electrical stimulation and refractory effects (Wilson et al., 1994). One way to improve signal representation is to reduce the level of across-fiber synchrony and hence, the level of the amplitude alternation. To accomplish this goal, we have examined EAP responses in the presence of Gaussian noise added to the pulse train stimulus. Addition of Gaussian noise at a level approximately -30 dB relative to EAP threshold to the pulse trains decreased the amount of alternation, indicating that stochastic resonance may be induced in the auditory nerve. The use of some type of conditioning stimulus such as Gaussian noise may provide a more 'normal' neural response pattern.

An improved method of reducing stimulus artifact in the electrically evoked whole-nerve potential

OBJECTIVE

Recording a compound action potential in response to electrical stimulation requires attention to minimize contamination due to electrical stimulus artifact. In patients implanted with the Nucleus 24 device, the electrically evoked whole-nerve potential (EAP) is recorded using a neural response telemetry (NRT) system. This system employs a forward-masking technique that greatly reduces stimulus artifact. However, theoretical considerations and experimental animal data suggest that the technique may distort the acquired EAP waveform under some situations. We proposed and evaluated a modification to the forward-masking technique that addresses this concern, particularly during collection of refractory recovery data.

DESIGN

We first examined neural responses of the electrically stimulated auditory nerve using cat preparations. Through single-fiber recordings from cats, we demonstrated underlying physiological limitations likely encountered with the "standard" forward masking technique. We then recorded feline EAP waveforms using both the standard technique and our proposed, modified, technique. Finally, we collected EAP data from human cochlear implant patients using both artifact reduction methods. These comparisons allowed us to evaluate the effectiveness of our modification.

RESULTS

The cat EAP data demonstrated that the standard forward-masking technique currently in use in the Nucleus NRT system can distort the EAP waveform when the nerve is partially refractory. In the cat, this distortion resulted in forward-masking recovery curves with artifactually prolonged recovery times and inaccurate latency trends. Similar effects were observed in the comparison of human recovery curves obtained using both the standard and modified techniques. In some cases, the modified technique produced EAP waveforms with more clearly defined peaks than were obtainable with the standard method.

CONCLUSIONS

Consideration should be given to implementing our modified forward-masking artifact reduction scheme, because it introduces less distortion of the EAP waveform and accordingly provides for more accurate assessment of the refractory properties of the electrically stimulated nerve.

Effects of time after deafening and implantation on guinea pig electrical detection thresholds

Changes in detection threshold level as a function of time after deafening and implantation have been described previously in macaque [Pfingst, 1990] and human [Skinner et al., 1995] cochlear implant subjects. Characterization of the mechanisms underlying these changes will contribute to our understanding of the anatomical and physiological factors affecting electrical stimulus detection. In addition, understanding the time course of early threshold changes is essential to the interpretation of acute physiological studies of cochlear implants. To better characterize time-dependent threshold changes, we monitored changes in guinea pig psychophysical electrical detection thresholds with time after deafening and cochlear implantation. Threshold levels for 100 Hz sinusoidal bursts were initially unstable over the first 30 days post-surgery (DPS), after which thresholds stabilized. At longer intervals (>100 DPS), increases (>10 dB) in threshold level were observed for 100 Hz sinusoids in three of 11 cases. These changes were transient in one case and long-term in two cases. The time course of threshold change, both early and late, could not be explained on the basis of changes in spiral ganglion cell survival. The guinea pig seems to be an ideal preparation for studies of this nature, because threshold changes are similar in type, but accelerated in time course, relative to those observed in primates.

Mutations in COL11A2 cause non-syndromic hearing loss (DFNA13)

We report that mutation of COL11A2 causes deafness previously mapped to the DFNA13 locus on chromosome 6p. We found two families (one American and one Dutch) with autosomal dominant, non-syndromic hearing loss to have mutations in COL11A2 that are predicted to affect the triple-helix domain of the collagen protein. In both families, deafness is non-progressive and predominantly affects middle frequencies. Mice with a targeted disruption of Col11a2 also were shown to have hearing loss. Electron microscopy of the tectorial membrane of these mice revealed loss of organization of the collagen fibrils. Our findings revealed a unique ultrastructural malformation of inner-ear architecture associated with non-syndromic hearing loss, and suggest that tectorial membrane abnormalities may be one aetiology of sensorineural hearing loss primarily affecting the mid-frequencies.

An empirically based model of the electrically evoked compound action potential

The relationship between electrically evoked single-fiber action potentials and the electrically evoked compound action potential of the auditory nerve is of interest to those attempting to model such responses with computational techniques. It also relates to efforts to exploit the gross potentials that can now be recorded by some implantable cochlear prostheses. In this paper, we develop a computational model of the auditory nerve response to single, pulsatile, electrical stimuli based upon the response characteristics obtained from 230 single fibers of 13 cats. These fibers were stimulated by brief (39s) monophasic cathodic stimuli delivered by a monopolar intracochlear electrode. The data were pooled to obtain an estimate of the distribution of fiber thresholds. Post-stimulus time histograms were modeled using Poisson functions and adjusted to account for empirically determined latency and jitter characteristics. The probabilistic nature of single-fiber input-output functions (i.e. Verveen's (1961) 'relative spread') was also modelled. PST histograms from 5000 modelled fibers were then summed and convolved with an estimated 'unit potential' following the method of Goldstein and Kiang (1958). This convolution produced modelled compound action potentials, which were then compared with experimentally obtained data. Manipulations of model parameters affecting threshold, jitter, and relative spread suggest that the most important determinant of the shape of the EAP amplitude-level function is the threshold distribution. A model based solely on threshold distribution produces an EAP input-output function similar to one that accounts for probabilistic single-fiber input-output functions. Discrepancies between these two models do occur if the threshold distribution function is compressed significantly, as might be the case in pathological cochleae with altered distributions or numbers of nerve fibers.

Electrically evoked single-fiber action potentials from cat: responses to monopolar, monophasic stimulation

We recorded action potentials from single auditory-nerve fibers of cats using monophasic current pulses delivered by a monopolar intracochlear electrode. These simple stimuli provided a means of investigating basic properties and hypotheses of electrical excitation. Standard micropipette recording techniques were used. Responses to anodic (positive) and cathodic (negative) stimulus pulses were recorded separately to evaluate stimulus polarity effects. Mean spike (action potential) latency was polarity dependent, with greater latencies for cathodic stimulation. Threshold stimulus level was also polarity dependent, with relatively lower cathodic thresholds. Both effects are consistent with trends reported in the compound action potential. Variability in single-fiber latency (i.e., jitter) was dependent upon stimulus polarity. In contrast, the slope of single-fiber input-output functions failed to show a clear polarity dependence, although such trends have been seen in the compound action potential data. We also observed a relatively greater degree of adaptation over time with anodic stimulation. Bimodal post-stimulus-time histograms were recorded in a small number (2%) of fibers, supporting the hypothesis that both the peripheral (dendritic) and central axonal processes are excitable with the same stimulus polarity, in a limited number of cases. This observation, together with analyses of interactions among measures of latency, threshold, and jitter, is consistent with the hypothesis that, with monopolar intracochlear stimulation, most fibers are stimulated at axonal (modiolar) sites and a minority of fibers nearest the electrode are stimulable at their peripheral processes.

Sensorineural hearing loss during development: morphological and physiological response of the cochlea and auditory brainstem

We have investigated the effects of sensorineural hearing loss on the cochlea and central auditory system of profoundly deafened cats. Seventeen adult cats were used: four had normal hearing; 12 were deafened neonatally for periods of < 2.5 years (five bilaterally, seven unilaterally); and one animal had a long-term (approximately 8 years) profound bilateral hearing loss. Bipolar scala tympani stimulating electrodes were bilaterally implanted in each animal, and electrically evoked auditory brainstem responses (EABRs) were recorded in an acute study to evaluate the basic physiologic response properties of the deafened auditory pathway. The cochleae and cochlear nuclei (CN) of each animal were examined with light microscopy. Spiral ganglion cell density in neonatally deafened cochleae was 17% of normal, and only 1.5% of normal in the long-term deaf animal. There was a 46% reduction in total CN volume in neonatally deafened animals compared to normal, and a 60% reduction in the long-term deaf animal. Neural density in the anteroventral CN of bilaterally deafened animals was 37% higher than normal; 44% higher in the long-term deaf animal. Significantly, however, we saw no evidence of a loss of neurones within the anteroventral CN in any deafened animal. There was a significant increase in EABR threshold and wave IV latency in the deafened animals, and a significant decrease in response amplitude and input/output function gradient. Again, these changes were more extensive in the long-term deaf animal. These data show that a sensorineural hearing loss can evoke significant morphological and physiological changes within the cochlea and auditory brainstem, and these changes become greater with duration of deafness. It remains to be seen whether these changes can be reversed following the introduction of afferent activity via chronic electrical stimulation of the auditory nerve.

Electrically evoked compound action potential (ECAP) of the cochlear nerve in response to pulsatile electrical stimulation of the cochlea in the rat: effects of stimulation at high rates

Some cochlear implant patients achieve better speech recognition with pulsatile electrical stimulation presented at high rates. The present study aimed to explore, in an animal model of cochlear implants, how the excitability of the cochlear nerve is affected by pulsatile electrical stimulation delivered at high rates, of up to 1,000-2,000 pulses per second (pps). Adult rats (n=23) were implanted with two or three stimulating electrodes in the left cochlea. In four of these rats, the left cochlea was deafened by local perfusion with 1 per cent or 4 per cent neomycin solutions prior to implantation. Pulsatile stimuli consisted of 20 micros electrical pulses, delivered in trains of 200 ms duration, separated by a pause of 200 ms. The pulse rates ranged from 100 to 2,000 pps (intra-train pulse rate). Electrically evoked compound action potentials (ECAPs) of the cochlear nerve were recorded either intracochlearly or from epidural electrodes (extra-cochlearly). With increasing pulse rates, the average ECAP amplitude decreased, whereas the average ECAP latency and its variability (SD) increased. For rates above 300 pps, the amplitude of the ECAP to the individual successive pulses delivered in the train progressively decreased during the initial part of the train, corresponding to a short-term adaptation of the cochlear nerve. This effect progressively increased for pulse rates ranging from 300 to 2,000 pps. In addition, there was a phenomenon of long-term adaptation, as indicated by a decrease in the amplitude of the ECAP to the first pulse of the train, indicating that the pause of 200 ms between each train was not long enough for full recovery of the cochlear nerve. This long-term adaptation was progressively more pronounced for increasing pulse rates. To characterize further the recovery in excitability of the cochlear nerve, forward masking experiments were conducted, showing a decrease of the ECAP amplitude when the interval between the first pulse (masker) and the second pulse (probe) was shorter than 2 ms. This ECAP decrease was slow for intervals between 2 and 1 ms and then abrupt for shorter intervals. The observations described above were similar for extra- and intra-cochlear recordings and were little, if at all, affected by treatment of the cochlea with neomycin.

Effects of electrode configuration on psychophysical strength-duration functions for single biphasic electrical stimuli in cats

The interface between electrode and neural target tissue is thought to influence certain characteristics of neural and behavioral responses to electrical stimulation of the auditory system. At present, the biophysical properties of this interface are not well understood. Here the effects of biphasic phase duration and electrode configuration on psychophysical threshold in response to electrical stimulation in cats are described. Five cats were trained to respond to acoustic stimuli using food as a reward in an operant reinforcement paradigm. After training, the animals were unilaterally deafened and implanted with a multicontact intracochlear electrode array. Thresholds for single presentations of biphasic current pulses were measured as a function of phase duration and electrode arrangement. Statistical analyses of the data indicated that strength-duration function slopes between 200 and 1600 microseconds/phase were significantly different for the different electrode configurations and, overall, were unrelated to the absolute level of the strength-duration function (i.e., were independent of absolute threshold). For all subjects, the slope of this function for intermediate pulse durations was dependent on electrode configuration and most shallow for radial-bipolar configurations (-3.4 dB/doubling), was steepest for monopolar arrangements (-5.9 dB/doubling), and was intermediate for longitudinal-bipolar pairings. (-4.4 dB/doubling). Slopes for both shorter and longer phase duration stimuli were not significantly different. The underlying mechanisms for these effects may include, or be a combination of altered electrical field patterns, integrated activity across multiple fibers, and stochastic behavior of individual auditory neurons to electrical stimulation.

Functional responses from guinea pigs with cochlear implants. II. Changes in electrophysiological and psychophysical measures over time

This study, the second of a two-part investigation, assessed changes over time in functional measures of the electrically stimulated auditory system following ototoxic deafening. Guinea pigs were trained to respond behaviorally to threshold level acoustic stimuli and then unilaterally deafened and implanted with a bipolar pair of electrodes within the cochlea and a single extracochlear electrode. Using pulsatile stimuli, thresholds for the electrically evoked auditory brainstem response (EABR) and psychophysical detection were repeatedly collected from the same animals over 3-month post-implantation periods. Thresholds were obtained as a function of stimulus phase duration primarily using bipolar intracochlear stimulation. As in earlier studies, the threshold measures exhibited both intra- and intersubject variability. Analysis of group data failed to show any statistically significant changes over time in either EABR or psychophysical threshold at any fixed pulse duration. However, significant changes over time were found in the slopes of the strength-duration functions for both measures. Slopes became shallower with time, suggesting a reduction in the efficiency of stimulus current integration, a trend presumed to occur with neural degeneration. This result suggests that strength-duration functions could be useful as a clinical diagnostic measure.

Electrically evoked auditory brainstem response in peripherally myelin-deficient mice

The integrity of the myelin sheath is important for normal electrophysiological function and survival of neurons that make up the auditory nerve. It is hypothesized that myelin deficiency of the auditory nerve may change the electrophysiologic characteristics of the auditory system, especially the temporal properties. In this study, the electrically evoked auditory brainstem response (EABR) was systematically evaluated in TrJ and Po-DT-A mice. Both of these mice have a deficit of their peripheral myelin. Correlation between the EABR and degree of myelin deficiency was also evaluated. The EABR in both strains of poorly myelinated mice exhibited prolonged latency, decreased amplitude, elevated threshold of wave I evoked by short-duration stimuli (20 microseconds/phase). A 2-pulse stimulation paradigm was used to evaluate refractory properties. Myelin-deficient mice exhibited slower recovery from the refractory state than controls. Long-duration stimuli (4 ms/phase) were used to assess integration properties. Myelin-deficient mice demonstrated prolonged wave I latency and more gradual latency changes with current level. Myelin thickness showed a strong correlation with EABR threshold for short-duration stimulation (r = -0.784), maximum wave I latency (r = -0.778) and the time constant of the wave I latency-current level function (r = -0.736) for long-duration stimulation and normalized wave I recovery functions (r = -0.718). These findings suggest that EABR measurement may be a promising tool to assess the electrically stimulated properties of auditory neurons, particularly related to the status of myelin sheath.